WG-KRILL-91/22

KRILL (EUPHAUSIA SUPERBA) DISTRIBUTION IN RELATION TO WATER MOVEMENT AND PHYTOPLANKTON DISTRIBUTION OFF THE NORTHERN SOUTH SHETLAND ISLANDS

T. Ichii, H. Ishii and M. Naganobu*

Abstract

In December/February 1990/91 a survey was conducted by RV Kaiyo Maru in the vicinity of the South Shetland Islands and Elephant Island. The main objective of the survey was to investigate the mechanisms responsible for the concentration of lain and to estimate lain biomass in the area surveyed. Krin biomass estimation was done by means of hydroacoustics. Phytoplankton distribution was assessed by measuring surface concentrations of chlorophyll a. Water circulation was studied with satellite-tracked buoys equipped with curtain drogues deployed at a level of 30 m from the surface because at this level lain concentrations are most frequently observed. Main lain concentrations were observed in shelf waters north of the islands as usual. Tracking of buoys demonstrated the existence of convergent complex eddies in shelf waters of these islands. A high concentration of chlorophyll a was also recorded in shelf waters. A spatial correlation between distributions of laill and chlorophyll a was observed. It is therefore considered that both hydrodynamic and food availability factors may be responsible for lain concentration. On a traditional lain fishing ground north of Livingston Island, laiU density had increased 3.4 times over a period of 40 days from late December 1990 to early February 1991. This increased krin density in early February 1991 (157 g/m2) was 54% lower than that observed in late January 1988. Information from studies of krill predators by US scientists also indicated that laill abundance during the fIrst half of the 1990/91 summer was lower than that in previous seasons. Total biomass of lain in shelf waters of the area surveyed was estimated at 1.78 million tonnes (56 g/m2) in late January 1991. The obtained estimate of laiU biomass should be regarded as the lower level for this time of the season. From mid-February and onwards it was reported that laill abundance had increased to the normal level.

Resume

En decembre/fevrier 1990/91, les alentours des lIes Shetland du Sud et Elephant ont fait l'objet d'une campagne d'evaluation a bord du RV Kaiyo Maru. L'objectif principal de la campagne etait l'investigation des mecanismes responsables de la concentration du krill et l'estimation de la biomasse du krill dans la region etudiee. C'est au moyen de methodes hydroacoustiques qu'a ete evaluee la biomasse du kriU. La repartition du phytoplancton a ete estimee en mesurant les concentrations de surface de chlorophylle a. Les mouvements de l'eau ont ete etudies a l'aide de bouees suivies par satellite et equipees d'ancres de cape, deployees a 30 m de profondeur, niveau auquel les concentrations de kriU sont le plus frequemment rencontrees. Comme de coutume, les principales

* National Research Institute of Far Sea Fisheries, 5-7-1, Orido, Shimizu, 424 Japan

123

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concentrations de krill ont ete observees dans les eaux du plateau nord des iles. La poursuite de bouees a revele l'existence de tourbillons complexes dteaux convergentes sur le plateau de ces iles. Une concentration elevee de chlorophylle a a egalement ete relevee dans les eaux du plateau. Une correlation spatiale a ete etablie entre la presence du krill et celle de chlorophylle a. I1 est ainsi possible dtenvisager que les facteurs hydrodynamiques et de disponibilite de nourriture soient responsables de la concentration du krill. Dans un lieu de peche traditionne1 de kriU, au nord de l'lle Livingston, la den site du krill a augmente de 3,4 fois en une periode de 40 jours de fin decembre 1990 a debut fevrier 1991. Cette den site accrue de kriU (157g/m2), debut fevrier 1991, etait de 54% inferieure a ceUe observee finjanvier 1988. Les informations provenant des etudes des predateurs de krill par les scientifiques des USA ont egalement indique que, pendant la premiere moitie de rete 1990/91, 1'abondance de kriU etait inferieure a celle des saisons precedentes. La biomasse totale de kriU des eaux du plateau de la region etudiee etait estimee a 1,78 millions de tonnes (56g/m2) fin janvier 1991. Cette estimation devrait etre consideree comme etant la plus faible pour cette epoque de la saison. A partir de mi-fevrier, l'abondance de kriU relevee avait augmente pour rejoindre le niveau normal.

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Resumen

De diciembre a febrero 1990/91, el BI Kaijo Maru realizo una prospeccion alrededor de las islas Shetland del Sur y Elefante. El objetivo principal consistio en investigar los mecanismos responsables de la concentracion de kriU y estimar la biomasa del krill en el area estudiada que se determino hidroacusticamente. La distribucion del fitoplancton se calculo por la c1orofila-a superficial. La circulacion del agua se estudio mediante el despliegue de boyas rastreadas por satelite, equipadas con unas "pantallas para medir corrientes" a una profundidad de 30 m de la superficie ya que las concentraciones de krill se encuentran mas frecuentemente a estas profundidades. Como es 10 usual, las concentraciones de krill mas importantes se encontraron en la plataforma, al norte de las islas. El rastreo de las boyas demostro la existencia de complicados torbellinos convergentes en las aguas de la plataforma de estas islas. Estas aguas tambien registraron una alta concentracion de c1orofila-a. Se observo una correspondencia espacial entre las distribuciones de krill y de clorofila-a, concluyendose asf que tanto la hidrodinamica como la disponibilidad de alimento son factores que afectan la concentracion de krill. En un caladero tradicional de krill situ ado al norte de la isla Livingston, la densidad de krill ha aumentado 3.4 veces en un perfodo de 40 dfas, de fines de diciembre de 1990 a principios de febrero de 1991. Este aumento de la densidad del krill a principios de febrero de 1991 (157 g/m2) representa una reduccion de un 54% en comparacion con aquella observada a fines de enero de 1988. Las investigaciones de cientfficos estadounidenses sobre los depredadores de krill, corroboraron el descenso en la abundancia de krill durante la primera mitad de la temporada 1990/91 en comparacion con temporadas anteriores. La biomasa total de krill en las aguas de la plataforma de la zona estudiada se calcul6 en 1.78 miUones de toneladas (56 g/m2) a fines de enero de 1991. Esta cifra de biomasa debiera ser considerada como el nivel inferior de la temporada. Desde mediados de febrero en adelante se informo que la abundancia de krill habfa aumentado a niveles normales.

1. INTRODUCTION

Krill concentrations regularly observed in the waters north of the South Shetland Islands and north and west of Elephant Island. Japanese krill fishing trawlers have mainly operated in this region with the yearly catches of 10 000 to 70 000 tonnes for the past eight years. This region, because of the abundance of krill-dependent predators, is also designated as an Integrated Study Region by CCAMLR.

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This krill-rich region is, however, only insufficiently known in regard to mechanisms responsible for high abundance of krill and interannual variability in its biomass. As basic information for these matters, this paper will present the distributional pattern of krill biomass in relation to water flow and chlorophyll a (ChI-a) distribution in 1990/91 austral summer.

2. MAlERIALS AND METHODS

The survey conducted in December/February 1990/91 by RV Kaiyo Maru was comprised of two legs. The first one covered the South Shetland Islands and Elephant Island regions including waters along the western side of the Antarctic Peninsula (Leg I: 22 to 29 December, 1990, Figure 1), whereas the second one covered thoroughly the waters from north of Elephant Island to north of the South Shetland Islands (Leg II: 18 January to 4 February, 1991, Figure 2).

2.1 Physical Oceanography and Chlorophyll a Studies

Oceanographic data were collected using a Sea-Bird SBE-19 CTD probe. CID stations were taken down to 1 000 m or to the sea bottom in shallow areas. Water samples were obtained from 11 depth levels (0, 10, 20, 30, 40, 50, 75, 100, 125, 150 and 200 m). Chl-a concentration was determined by fluorometric method with a Turner Model-Ill fluorometer after extraction with 90% acetone (Strickland and Parsons, 1972).

2.2 Hydroacoustic Studies

Hydroacoustic studies were carried out in order to determine (i) the overall distribution of krill biomass in the survey area; and (ii) changes in krill abundance which took place in time between Leg I and Leg II on krill fishing grounds north of Livingston Island (shown as shaded areas in Figures 1 and 2). Observations were conducted along closely-spaced transects (4-mile intervals). The fact that Leg II of the survey was carried out almost in the same area as the previous cruise of RV Kaiyo Maru in 1988 (Fisheries Agency, 1989), made it possible to observe interannual variability in krill biomass in this area.

The echo sounder used was FQ-50 with a digital integrator (Furuno Electric, Japan). Operating frequency was 200 kHz. Operational characteristics of the acoustic system is summarised in Table 1. Throughout the whole survey, excluding time spent for station measurements and net towing, the mean volume back-scattering strength (MVBS) was measured for constant horizontal integration intervals of 1 n mile and the depth range from 10 to 200 m or to the bottom if shallower. The top level of the integration depth, 10 m, was changed sometimes to 20 m in order to avoid impact of the surface noise. The default value of the target strength of krill was -66.1 dB per kilogram wet weight of krill (Shimadzu et al., 1989). This value was obtained by Shimadzu et al. (1989) who measured krill of about the same size as observed during the present survey. Before (29 October 1990) and after (26 March 1991) of the cruise, the echo-integrator was calibrated in the port of Tokyo with the help of a hydrophone. Two calibrations showed a little reduction in the source level and receiving sensitivity of the echo-integrator after the cruise.

2.3 Drifting Buoy Studies

For the study of flow patterns around the islands, five drifting buoys (model C-2243, TOKYOCOM, Japan) were released in each Leg (Figures 1 and 2) and tracked using the Argos system carried on TlROS-N and NOAA-A satellites. Typically, twelve locations per day were obtained for each buoy with an accuracy higher than several hundred metres. Since krill in this region tended to be most abundant within the depth range of 30 to 40 m (Fisheries

126

Agency, 1989; observations of the 1987/88 austral summer), each buoy was equipped with a curtain drogue (4 m x 1 m) deployed at 30 m depth in order to study flow patterns at this depth.

3. RESULTS

3.1 Physical Oceanography

Figure 3 shows the horizontal distribution of salinity at 30 m depth during Leg II. Waters in the study region are divided into three categories, i.e. oceanic, frontal and shelf waters. The frontal waters, which are called CWB (Continental Water Boundary), are characterised by a relatively intense southward increase in salinity (from 33.7 to 34.1 ppt) and are restricted to a narrow band near the continental slope.

3.2 Chlorophyll a

High concentration of surface Chl-a was observed in the shelf waters with the richest area (higher than 1.0 mg/m3) located to the north of Livingston Island (Figure 4). High gradients of Chl-a concentration were observed over the frontal waters with concentration values decreasing northward. Chl-a concentration was very low (0.2 to 0.3 mg/m3) in the oceanic waters.

3.3 Hydroacoustic Surveys

Mean krill density was calculated for each distance between stations along the cruise track (Figures 5 and 6). In Leg I krill tended to form small or dispersed swarms with the biomass of 22 to 76 g/m2 along the Antarctic Peninsula and 16 to 123 g/m2 near Livingston Island. The highest value (123 g/m2) was observed in the shelf waters north of Livingston Island. Krill were absent from a large part of the survey region. The fact that one krill fishing trawler operated at that time not only to the north of Livingston Island but also to the north of King George Island implies that the acoustic survey failed to detect krill to the north of King George Island, probably because krill occurred in widely spaced small swarms. On the traditional fishing ground on the northern shelf of Livingston Island, where the intensive acoustic survey was undertaken, the average biomass of krill was 46 g/m2 on 25 December, 1990.

During Leg II the acoustic survey revealed a distinct tendency of high krill abundance observed in shelf waters, low abundance in oceanic waters and intermediate abundance in frontal waters. Higher values of biomass (> 100 g/m2) were most frequently observed to the north of the South Shetland Islands with the highest biomass of 397 g/m2. In oceanic waters, where krill were almost absent during Leg I, biomass of approximately 10 g/m2 was observed during Leg n. On the traditional fishing ground the average biomass was 157 g/m2 on 3 February, 1991.

To obtain the total biomass throughout the Leg II survey area (defined by broken lines in Figure 6), the area was divided into five subareas according to the regularity in kriU density distribution. Krill biomass was calculated for each subarea. The total biomass, the sum of the biomass calculated for each subarea, was 2.11 million tonnes, of which 1.78 million tonnes (157 g/m2) was concentrated in shelf waters (subareas C, D and E).

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3.4 Drifting Buoy Studies

Seven buoys continued to operate for more than 50 days, while the other three failed shortly after deployment. Figure 7 shows tracks of four buoys.

Buoy 1 deployed in oceanic waters moved northeast with meandering and eddying current and arrived at South Georgia region five and a half months after deployment. The buoy appeared to drift in the Weddell-Scotia Confluence from west of Elephant Island onward. It was retrieved west of South Georgia, where Russian and Japanese trawlers were conducting winter krill fishing. Another buoy released in oceanic waters in Leg I (not shown in Figure 7) drifted eastward between Elephant Island and King George Island, and was trapped by erratic oceanic eddies to the east of Elephant Island. This buoy, in the end, became entrained in waters adjacent to Elephant Island.

Buoys 2 and 3 drifted eastward in the northern and southern portions of frontal waters, respectively. The latter one, however, recurved landward, then became entrained within the eddy north of Livingston Island and for a duration of one and half circulations before exiting it. The buoy left the eddy on the day when a furious storm occurred (wind scale was 10 by Beaufort scale). The buoy further travelled round the South Shetland Islands anticlockwise (with smooth current on the southern side of the islands and erratically rotating current on the northern side) and, surprisingly, executed more than one and a half circulations around the islands before ceasing to transmit. The track of this buoy elucidated the occurrence of complex eddies in the waters north of the South Shetland Islands. As for Buoy 2, it continued to drift eastward, then became entrained in the waters adjacent to Elephant Island, and went round the island anticlockwise while moving in loops along both the northern and western sides of the island.

Buoy 4 deployed in the shelf area moved east, then reversed and was trapped in Barclay Bay on the northern side of Livingston Island. It stalled there for more than seven months.

Thus, the buoys exhibited a distinct tendency to be trapped in complex topographical eddies generated in shelf waters.

4. DISCUSSION

Major concentrations of krill were observed in shelf waters north of the South Shetland Islands and Elephant Island, which mainly corresponds with the general pattern of krill distributions expected for the summer season. This study suggests two factors responsible for krill concentration. One is a convergent accumulation of krill by complex topographic eddies. Because of its rather stationary nature, the topographic eddies may provide some spatially-stable sites for retention of krill swarms, leading to the formation of persistent concentrations. Another factor is a behavioural response of kri11. High concentration of Chl-a was observed in shelf waters. Krill arriving to near the islands would detect and respond to concentration gradient of phytoplankton (Price, 1989) and as the result would congregate actively in shelf waters. It is reasonable to assume that these two factors worked together, and resulted in a marked increase of krill abundance over the period from Leg I to Leg IT (Figures 5 and 6).

Buoy 1 travelled all the way to South Georgia and has been trapped on krill winter fishing grounds. This implies that krill can reach their favourable habitats (e.g., island regions) even by passive drifting only. Considering the observed strong current-borne movement of krill (Everson and Murphy, 1987), krill may make much use of these currents for migration.

128

Krill abundance increased 3.4 times on the traditional fishing ground north of Livingston Island over the 40-day period during 25 December 1990 to 3 February 1991. This increase rate is quite comparable with 5-fold increase observed around Elephant Island over the two-month period from January to February in 1990 (Amos et al., 1990).

The estimate of krill abundance (157 g/m2) obtained for this traditional fishing ground on 3 February 1991 was approximately half of the estimate (342 g/m2) obtained there on 20 January 1988 (Fisheries Agency, 1989). The distributional pattern of krill was also different between these seasons: in 1991 'layers' (a form of krill swarm with horizontal length exceeding 1 000 km) were observed only occasionally, whereas in 1988 'layers' were the main feature (Endo and Shimadzu, 1989). Brinton (1987) reported similar differences between March 1981 and March 1984 in biomass (229 g/m2 vs 134-201 g/m2) and distributional pattern (large, thick swarms vs smaller, dispersed swarms) of krill near Elephant Island.

The us AMLR report (1991) described the 1990/91 season as characterised by the following features: krill abundance was low (compared with other seasons) during the first half of summer (December to early February), whereas it approximated to the normal level towards the end of summer (mid-February to mid-April). On Seal Island, low krill abundance in the beginning of this summer caused approximately 20% decline (as compared to last year) in numbers of penguins occupying nests, and made feeding trips of fur seals rather long (5 to 9 days compared to 1 to 3 days later in the season) (AMLR, 1991). Krill fishery data from the 1990/91 season have also indicated that density index of krill (catch-per-hour of fishing) remained low in January. Therefore, the estimate of total krill biomass throughout shelf waters in late January 1991, i.e., 1.78 million tonnes (157 g/m2) , should be regarded as the lower level of krill biomass for this region at this time of the season.

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ACKNOWLEDGMENTS

We are grateful to Captain Morooka T., officers and crew of RV Kaiyo Maru for their assistance during the cruise.

REFERENCES

AMLR. 1991. United States Antarctic Marine Living Resources 1990/91 Field Season Report. ROSENBERG, J. and R. HEWITT (Eds). 97 pp.

AMOS, A.F., J.L. BENGTSON, 0 . HOLM-HANSEN, V .J. LOEB, M.C. MACAULAY and J.H. WORMUTH. 1990. Surface water masses, primary production, krill distribution and predator foraging in the vicinity of Elephant Island during the 1989/90 austral summer. Document WG-CEMP-90111. CCAMLR, Hobart, Australia: 65 pp.

BRINTON, E., V.J. LOEB, M.C. MACAULAY and E. SHULENBERGER. 1987. Variability of Euphausia superba populations near Elephant Island and the South Shetlands: 1981 vs 1984. Polar Bioi., 7: 345-362.

ENDO, Y. and Y. SHIMADZU. 1989. Size and density of krilllayers fished by a Japanese trawler in the waters north of Livingston Island in January 1988. In: Selected Scientific Papers, 1989 (SC-CAMLR-SSP/6). CCAMLR, Hobart, Australia: 339-344.

EVERSON, I. and E. MURPHY. 1987. Mesoscale variability in the distribution of krill, Euphausia superba. Mar. Ecol. Prog. Ser. 40: 53-60.

Fisheries Agency of Japan. 1989. Report of Kaiyo Maru's fifth Antarctic cruise, March 1989. pp 301. (In Japanese).

PRICE, H.J. 1989. Swimming behaviour of krill in response to algal patches: A mesocosm study. Limnol. Oceanogr., 34(4): 649-659.

SIDMADZU, Y., T. KOIKE and T. SUGURO. 1989. Target strength estimation of Antarctic krill, Euphausia superba by cooperative experiments with commercial Trawler. Document SC-CAMLR-V//IIBG/30. CCAMLR, Hobart, Australia: pp 10.

STRICKLAND, J.D.H. and T.R. PARSONS. 1972. A practical handbook of sea analysis. Bull. Fish. Res. Board Can., 167: 311.

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Table 1: Operational characteristics of the echosounder Furuno FQ-50.

Frequency 200 kRz

Equivalent beam width 0.007 sr

Pulse duration 1.8 ms

Depth range Ot0200m

Depth channel 10* to 200 m (9 channels)

Integration interval 1 nautical mile

Attenuator 20 dB

Threshold 15 dB

TVG 20 log R

Gain constant 78.9 dB

* The level of the top depth integration range was changed to 20 m in order to avoid impact of the surface noise when the sea was rough.

131

60· S

61

62

63

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LEG I 22-28 Dec. 1990

R/V KAIYO MARU

Intensive acoustic survey area

x: Dep I oyment po of Argos buoy

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132

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60

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18 Jan. - 2 Feb. 1991

Salinity - 10m

t' , 33.8

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Figure 3: Distribution of salinity at 10 m depth in Leg H.

134

LEGII 18 Jan. -4 Feb.1991

R/V KAIYO MARU

Ch I • ..Q. Cmg/m3) - Om

Figure 4: Distribution of chlorophyll a (mg/m3) at the surface in Leg IT.

54°W

135

LEG I 22 - 29 Dec. 1990

RIV KAIYO MARU

:Intensive acoustic survey area D

>130 g;,n2 100g/O1 2 50 75

10 25 5

0.5>

Figure 5: Hydroacoustically detected laill distribution in Leg I.

136

60' S

61

62

63

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LEGll 18Jan,-2 Feb.1991

R!V KAI YO M ARU A

...... \ ...... \

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\'> ......... cS 0 0 ° ° ·:: ... ·0....... 0/ E

...... /\.O·~······ 0 0 0 ···~O····· 0 ..... 9; ... 0" '. 0 0 ......... 0 ". \ ( .... ,'." 0 .... 0 .' . ° \ • ... '. ····0 ...... ° 0 0.0 \ '. 0 '. -

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Figure 6: Hydroacoustica11y detected laill distribution in Leg n.

137

24 Dec.1990-25 May 1991

R/V KAI YO M ARU

Circulation at 30-35 m deJth

54°W

Figure 7: Water circulation at 30 to 35 m depth derived from the paths of four satellite­tracked buoys. Since Buoy 3 drifted more than one and a half circulations around the South Shetland Islands, its second circulation is shown by dotted line.

138

Liste des tableau

Tableau 1: Caracteristiques operationnelles de l'echo-sondeur Furuno FQ-50.

Figure 1:

Figure 2:

Figure 3:

Figure 4:

Liste des figures

Trajet de la campagne et stations du leg I.

Trajet de la campagne et stations du leg n. Distribution de la salinite a 10 m de profondeur dans le leg n. Distribution de la chlorophylle a (mg/m3) en surface dans le leg n.

Figure 5:

Figure 6:

Figure 7:

Distribution du krill decele par methode hydroacoustique dans le leg I.

Distribution du krill decele par methode hydroacoustique dans le leg n.

Mouvements de l'eau a une profondeur de 30 a 35 m, derives des deplacements de quatre bouees suivies par satellite. Vu que la bouee 3 a derive plus d'une fois et demie autour des lles Shetland du Sud, son deuxieme parcours est illustre par la ligne pointillee.

CnHCOK Ta6JIHIJ;

Ta6JIHIJ;a 1: Pa60lme xapaKTepHcTHKH 3XOJIOTa cI>YPYHO FQ-50.

CnHCOK PHCYHKOB

PHCYHOK 1: MaplIIpYT nJIaBaHH5.I H CTaHIJ;HH (oTan n.

PHCYHOK 2: MaplIIpYT nJIaBaHH5.I H CTaHIJ;HH (oTan 11).

PHCYHOK 3: Pacnpe.lleJIeHHe COJIeHOCTH Ha rJIy6HHe 10 MeTpoB (oTan 11).

PHCYHOK 4: Pacnpe.lleJIeHHe XJIOpocpHJIJIa a (Mr/M 3) Ha nOBepXHOCTH (oTan IIl.

PHCYHOK 5: Pacnpe.lleJIeHHe KPHJI5.I, o6HapY:>KeHHOe MeTO.llOM rH.lIpoaKycTHKH (oTan

n.

PHCYHOK 6: Pacnpe.lleJIeHHe KPHJI5.I, o6HapY:>KeHHOe MeTO.llOM rH.lIpOaKycTHKH (oTan II).

PHCYHOK 7: UHPKYJI5.lIJ;H5.I BO.llbI Ha rJIy6HHe 30-35 M., paCC4HTaHHa5.1 no KypcaM

4eTblpex 6yeB nYTeM cnYTHHKOBoro CJIe:>KeHH5.I. IIOCKOJIbKY Byti 3 cOBeplIIHJI BOKpyr IO:>KHbIX IIIeTJIaH.lIcKHx OCTPOBOB 60JIblIIe 4eM 1.5 IJ;HPKYJI5.lIJ;HH, ere BTOpa5.1 IJ;HpKyJI5.lIJ;H5.I OTMe4eHa nYHKTHpHoti JIHHHeti.

139

Tabla 1:

Figura 1:

Figura 2:

Figura 3:

Figura4:

Figura 5:

Figura 6:

Figura 7:

140

Lista de las tablas

Caracteristicas operativas de la ecosonda Furuno FQ-50.

Lista de las figuras

Derrota y estaciones realizadas en la primera fase.

Derrota y estaciones realizadas en la segunda fase.

Distribuci6n de la salinidad a 10 m de profundidad en la segunda fase.

Distribuci6n de la c1orofila-a (mg/m3) superficial en la segunda fase.

Distribuci6n de kriU determinada hidroacusticamente en la primera fase.

Distribuci6n de kriU determinada hidroacusticamente en la segunda fase.

Circulaci6n del agua de 30 a 35 m de profundidad, deducida del trayecto de cuatro boy as rastreadas por satelite. Ya que la boya No 3 fue impulsada por la corriente mas de una vez y media alrededor de las Shetlands del Sur, su segundo trayecto esta marc ado por una linea punteada.